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Fair Value of Financial Instruments, Assets and Equity Investments (Tables)
 9 Months Ended
Sep. 30, 2015
Fair Value Disclosures [Abstract]  
Fair Value and Carrying Value of Long-Term Debt
The fair value and carrying value of our senior notes were as follows: 
(Dollars in millions)
September 30, 2015
 
December 31, 2014
Fair value
$
5,675

 
$
6,733

Carrying value
6,259

 
6,660